Allow for "_[_" and "_]_" in docs

* hydra.el (hydra--format): Update.

* hydra-test.el (hydra-format-6): Add test.

Fixes #143

hydra-test.el

@@ -1096,6 +1096,23 @@ _f_ auto-fill-mode:    %`auto-fill-function
             #("u" 0 1 (face hydra-face-red)))
             ""))))
 
+(ert-deftest hydra-format-6 ()
+  (should
+   (equal (hydra--format
+           nil nil "\n[_]_] forward [_[_] backward\n"
+           '(("]" forward-char)
+             ("[" backward-char)))
+          '(concat
+            (format
+             "[%s] forward [%s] backward\n"
+             #("]"
+               0 1 (face
+                    hydra-face-red))
+             #("["
+               0 1 (face
+                    hydra-face-red)))
+            ""))))
+
 (ert-deftest hydra-format-with-sexp-1 ()
   (should (equal
            (let ((hydra-fontify-head-function

hydra.el

@@ -513,7 +513,7 @@ The expressions can be auto-expanded according to NAME."
         offset)
     (while (setq start
                  (string-match
-                  "\\(?:%\\( ?-?[0-9]*s?\\)\\(`[a-z-A-Z/0-9]+\\|(\\)\\)\\|\\(?:_\\( ?-?[0-9]*?\\)\\([-[:alnum:] ~.,;:/|?<>={}*+#]+?\\)_\\)"
+                  "\\(?:%\\( ?-?[0-9]*s?\\)\\(`[a-z-A-Z/0-9]+\\|(\\)\\)\\|\\(?:_\\( ?-?[0-9]*?\\)\\(\\[\\|]\\|[-[:alnum:] ~.,;:/|?<>={}*+#]+?\\)_\\)"
                   docstring start))
       (cond ((eq ?_ (aref (match-string 0 docstring) 0))
              (let* ((key (match-string 4 docstring))